Cloud-radiative interactions with the North Atlantic storm track

Project S6 is performed by the junior research group "CONSTRAin: Cloud-radiative interactions with the North Atlantic storm track" that was established for the second phase of HD(CP)². Within HD(CP)² we articulate the coupling of small-scale cloud processes with the large-scale circulation of the extratropical atmosphere. We study how diabatic cloud processes interact with and shape the extratropical circulation and its response to climate change, with a particular focus on the North Atlantic storm track and the role of cloud-radiative effects. To this end we apply the new ICON atmosphere model over a hierarchy of spatial resolutions and setups, and combine these simulations with the analysis of observations and theoretical approaches.

Although one of the most frequently studied components of the climate system, the response of the mid-latitude storm tracks to climate change remains subject to large uncertainties that hinder planning of effective climate adaptation strategies. Recent work suggests that a substantial part of these uncertainties are caused by cloud-radiative effects, which, due to the coarse resolution of global climate models, need to be parametrized. An example is given in the figure below, which shows idealized aquaplanet simulations with the ECHAM6 atmosphere general circulation model. In these, cloud changes cause half of the simulated change in the extratropical winds and poleward shift of the jet stream. However, the impact of clouds on the dynamics of the extratropical atmosphere are not well understood and are poorly constrained. This gap in knowledge provides our scientific motivation.

Idealized aquaplanet (no land) simulations with the ECHAM6 model and with prescribed sea-surface temperature. Global warming is mimicked by a uniform 4 K surface warming.
Left: global warming leads to a dipole change in near-surface zonal winds, indicating a poleward shift of the mid-latitude jet stream and storm tracks (black). Radiative changes of clouds alone cause about half of the wind change (red), as is shown by simulations with prescribed clouds and water vapor.
Right: the cloud impact on the circulation results from the radiative forcing that is generated by the cloud response to global warming. Understanding how the different spatial components of the complex radiative forcing pattern affect the circulation is one of the goals of the storm track group. The figure is adapted from Voigt and Shaw 2016 (Journal of Climate) and Shaw et al. 2016 (Nature Geoscience).

The questions we aim to answer include timescales ranging from days to centuries, and spatial scales ranging from the North Atlantic to the global scale. We are responsible for designing the North-Atlantic ICON_LEM simulations at the large-eddy resolving scale (WP1). We work on the role of clouds for the evolution of individual mid-latitude cyclones and cyclone spectra (WP2, WP6), dynamic and thermodynamic controls on clouds and their implications for short-term circulation variability (WP3), and the role of local and remote cloud changes for the response of jet streams and storm tracks to long-term climate change (WP4). At a later stage of the project we also plan to lead an international model comparison project on the cloud-circulation coupling (WP5), and to research ways to incorporate clouds into idealized climate models to foster the link between simulations, observations and theories of the extratropical circulation (WP7). The overarching aim of our work is to determine the level of cloud representation that is warranted in global climate models from the perspective of large-scale atmospheric dynamics and climate dynamics, and to what extent this level depends on the component of the extratropical circulation being investigated and the specific question being asked (WP8). Our hope is that this will also allow us to better judge the plausibility of the wide spectrum of future circulation changes predicted by current global climate models, to rule out some of the model behavior, and to better quantify future changes. This vision is reflected in our acronym CONSTRAin.

We are based at the Institute for Meteorology and Climate Research of Karlsruhe Institute of Technology. We maintain and develop active collaborations within Germany as well as internationally, including, for example, with researchers from Columbia University, University of Chicago and the Laboratoire de Meteorologie Dynamique.